Reach for the Stars

This will likely be the longest grind, even after doing NG+ and the DLC - its a long road to level 100.

Make sure to max affinity with a companion and get rested by sleeping for the +15% XP boost (vs +10% without a maxed companion), and look at the consumables/beverages you can buy or craft for extra XP boosts (like Tranquilitea or Alien Tea).

Also look at the game's difficulty settings; increasing combat difficulty, or reducing access to ship's cargo to only when you're on the ship, and various other settings all add incremental XP boosts. Its worth turning all these up to as high as you can stand for +40% or so XP in everything you do. If its too challenging, you can generally raise/lower these settings whenever you want without any particular penalty.

You can also consider setting up outputs (which you'll need for Home Sweet Home and Industrialist anyway) and have them manufacture basic resources, store them in storage units and then you can bulk-craft cheap items; e.g. if you can mine Iron and Aluminium in the same outpost then bulk-crafting Adaptive Frames can be lucrative. There are more advanced setups, but balancing the time taken to set up outposts and the links between them is counter-productive.

Alternatively, fly about and blow people up - the Freestar Rangers mission board in Akila City is a good place for quick missions to help do this.